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# Question about Capacity of 200amp Main

## Question about Capacity of 200amp Main

#1
08-28-14, 06:10 AM
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Question about Capacity of 200amp Main

We are considering adding a second washer + dryer + range to our house (in the basement) and I'm trying to figure out what risk I have of blowing the 200A main.

Also to note: I have a wood shop in my 2 car garage with several power hungry tools - like table saw, dust collector, shop vac, bandsaw, etc.

So the first big question I have is basic and simple:
Assume I have a light bulb that consumes 20A @ 120v. How many light bulbs can I turn on before tripping the 200A main?

Depending on the response to the question above, I will ask more detailed questions.

#2
08-28-14, 06:13 AM
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perhaps another way to word the question, is "Is the 200A Main @ 240V or 120V?"

#3
08-28-14, 06:31 AM
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You need to do a load calc first. Here is one example but an online search should show more: http://www.docstoc.com/docs/97696973...lt-Enterprises.
perhaps another way to word the question, is "Is the 200A Main @ 240V or 120V
Your house is supplied with 240 volts so it is 240 volts. (120 volts is a derived voltage from either of the 240 legs using the neutral, a center tap on the secondary of the transformer on the pole or pad.)

#4
08-28-14, 06:54 AM
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200A * 240V = 48000 VA
20A * 120V = 2400 VA
48000/2400 = 20 light bulbs should run forever without tripping breaker. The 21st light bulb would not trip it immediately. Could take minutes, an hour, or never as the tolerances are not that precise.

A load calculation is the standard way of figuring out what the main should be. They take into account the likelihood of what will be on at the same time and for how long.

#5
08-28-14, 07:49 AM
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Great, that answers my question completely well. Thank you!

A few days ago, a friend and I were working in the garage. I noticed I had the shop vac & bandsaw running, and he had the table saw & dust collector running. The garage is on a subpanel and the tools are split up on diff circuits well. Then I realized.... oh no, what if in the future, we're doing this while my wife's cooking, doing laundry, and tenants downstairs are also cooking & doing laundry? I'd hate to trip the whole thing.

But from your answer, I'm thinking that we should be safe. I'll do a load calc as Ray suggested. Thanks!

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